8ch cheap (chip) amplifier for sound art

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.

NeoY2k

Member
2007-12-27 2:53 pm
Hello,

I'm looking to build an 8 channels amplifier for sound art (installations, theatre...) and would appreciate some input as to best suited design. The goal is to have an ultra inexpensive and small multichannel amplifier.

Inexpensive is easily achieved stacking 20-30$ used hifi amplifier, but that's not fun to carry around. 4ch Behringer amps for around 350$ would work but multiply that by 2 and it's not cheap enough, especially as I plan to upgrade to 16 ch (make a second unit) within a year.

Requirements are:
-mid low power (50W-80W RMS 8R @ 0,1% THD)
-reasonable peak power, ability to have higher output for a couple seconds.
-driving all sort of loads, tactile transducers, small hifi speakers, computer speakers...
- some sort of OL/short/whatnot protection
- decent noise floor
- As cheap as possible.
- Reliable
- Compact/light

My first thought is buying a handful of LM3886 PCBs off ebay for dirt cheap, and stuff them on a large heatsink. Power them with a Meanwell SMPS. Would love to find a way to stuff them in a small 1u rack, and would absolutely love to have passive cooling (goes against light, I know).

But I'm concerned by the lack of balanced input and gain adjustment (strapping a pot on the input resistor is calling for high noise floor at lower gains), I'm worried that I won't find suitable symmetrical SMPS (and I would need to make my own PCB for single rail it would seem). I might want a tad more power.

So, would there be a better solution?

I had a look at the 6*100W Sure Electronics boards, but reliability seems a concern, with overheating. So, not suited. I also like the fact that the LM3886 are mono amps: stuff as many as needed, if one fries, easily replaced/repaired. I won't be treating the amp nicely xD.

Note: I can design PCBs and have them made, but Olimex is not taking orders anymore and it was the cheapest I could find. And I would have had trouble making them for <10$ per channel anyway. So this probably will be a too expensive route.

Thanks,
Nicolas
 

samp78

Member
2011-03-07 4:43 pm
Hi Guys, This is the most appropriate forum I think for what I am looking for... I am looking at building a home cinema system so 5.1 or 7.1 haven't decided yet... I was going to use LM3886 for the 5-7 channels and a bridged LM3886 for the sub. My question is what skinda power will this draw i.e. what VA transformer will i require? looking at running +-30V rails
Thanks in advance
 
Let me suggest you take a look at the power requirements of the speakers you will be using and take your lead from that information (working backwards). It has been my experience and understanding the Class D amps are better suited to subs. My multiple LM3886 amps improve their ability to handle 4 ohm drivers but don't necessarily create more power/SPL.
 

samp78

Member
2011-03-07 4:43 pm
Okay well it will be a mono sub only 10" not after moving massive amounts of air more like bass reinforcement that sub-woofing, but it is 8 ohm, I do have 2 to hand though so could parallel them to make 4 ohms? was contemplating Class D but I can get hold of LM3886's cheaper and more readily... and the satellites will be 6 Ohm Sony surround speakers I have managed to get my mits on :) not very good at the power side of electronics, i know the basics but some things just confuse me... Thanks
 
Hi Guys, just a quick question can the LM4780 run a 4 ohm load at rail voltages of around 32V?

Yes, pretty much. You are somewhere around the comfortable limit and I'd want to know which side of 4 ohms your load actually is, how much heatsinking you are using and how loud you like your music.

If you are putting both halves of the 4780 in parallel, then it's a breeze and you could go up in Voltage.

But, for the most part, you are almost certainly going to be OK.
 
Ignore AndrewT. You'll be fine.

The chip has a current limit at 11A - let's call that 7A - and a voltage range of 84V. Both identical to the 3886. You are not going to go near either, realistically. Moreover, with a standard linear supply the voltage will sag significantly if it's called on to produce anything nearby, wiping off a few 10s of theoretical watts available.

What it doesn't have is twice the area to dissipate its own heat - hence my wanting to know that you would properly heatsink it (ie a 10cm x 10cm x 12mm slab of heatsink will not be meaty enough) and wanting to know how hard you are going to push it and into what DCR (which you can measure with any multimeter). Unless the bass unit has a DCR of 3.3 ohms - so getting just inside the IEC definition of 4 ohms - you will be fine.

Besides, I have not heard of these chips failing through 'wearing out'. There are examples of them failing in things like guitar amps (yes they are used in that role) where they have little in the way of heatsinks and are forced to be 100W amps driving extraordinarily uncertain loads - like as many speakers the user can parallel up to get it as loud as the headbanger wants.

Does anyone else disagree - or have a reason to think that they might?
 

samp78

Member
2011-03-07 4:43 pm
Thanks Christian, I will probably be pushing them pretty hard, but not using them for guitar amplification that's another project all together! And not sure what the DCR is at the moment as I need to check this and the drivers aren't at my house. what sort of size heatsink will it require if I was to use 2 8 ohm drivers in parallel? Thanks
 
what sort of size heatsink will it require if I was to use 2 8 ohm drivers in parallel? Thanks

Two 8 ohm drivers in parallel will have a DCR of about 3.4 ohms almost certainly (ie lower than a single 4 ohm unit is likely to be) so you are again chomping at the boundaries.

In general I would like to see at least twice the area mentioned above, and probably at 16mm or 19mm. On top of that I would like to see the dissipation area of some casework as well. This last part can be really quite important. It doesn't have to be thick but it is worthwhile having decent joints that will conduct heat. At the very least have a panel that is perhaps between the chip and the heatsink and with liberal quantities of thermal paste. Incidentally, vertical mounting of the heatsink (so air can flow upwards through it) is significantly more efficient than horizontal mounting.
 

samp78

Member
2011-03-07 4:43 pm
So instead of bridging the LM4780 if i ran it in stereo config with the 2 8 ohms drivers, and then just run the mono signal in, would this be better? and could I use some kind of ventilation system ie fans? I can get hold of a lot of CPU heatsinks... Thanks for all your help everyone :)
 
So instead of bridging the LM4780 if i ran it in stereo config with the 2 8 ohms drivers, and then just run the mono signal in, would this be better? and could I use some kind of ventilation system ie fans? I can get hold of a lot of CPU heatsinks... Thanks for all your help everyone :)

Where are you going with this? Bridging is the last thing you want to do at this voltage. Then all of AndrewT's dire predictions will certainly come true!

You can run it with an 8 ohm driver each side, if you like, and that's a breeze. You can run it with 4 ohms drivers each side and it will manage, with some care. That also probably holds true for two 8 ohm drivers in parallel as well, but more care will be required - as outlined in my last response. Active cooling will inevitably help as mentioned above.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.